A well-known method of evaluating the quality of MCMB is an analysis using solvents. This method evaluates the quality of MCMB using, as indices, the percentages (by weight) of toluene-insoluble fraction (TI), quinoline-insoluble fraction (QI) and .beta.-resin [TI-QI].
However, for the quality evaluation of MCMB, this method is not wholly satisfactory. While what matters in the application of MCMB is the flexural strength of sintered bodies obtainable by sintering the MCMB at a temperature not below 1,000.degree. C., there actually exist MCMB species showing substantially the same TI, QI and .beta.-resin percentages and yet giving widely different flexural strength values after sintering at 1,000.degree. C. or above. The reason for this variation is presumably as follows. The binding component of MCMB is so reactive and susceptible to oxidation that the MCMB is ready to undergo oxidative degradation. Compared with unoxidized MCMB, MCMB that has undergone low-temperature oxidation yields a considerably lower flexural strength value after firing at 1,000.degree. C. or above, even though its TI, QI and .beta.-resin contents remain almost unchanged from the unoxidized MCMB.
The only reliable method available today for the quality evaluation of MCMB comprises taking a sample of MCMB from a lot in question, subjecting it to pressure-molding under a given pressure load, sintering the molding at 1,000.degree. C. and measuring the flexural strength of the sintered body. No method is available for predicting the characteristics of a 1000.degree. C.-sintered body from the characteristics of raw MCMB.
Furthermore, in some instances, the pressure-molded MCMB swells in the course of firing up to 1,000.degree. C., thus failing to give satisfactory sintered products of practical value. There is no method, either, for prognosticating, based on the characteristics of unprocessed MCMB, whether such swelling would occur or not.